Tappet in an internal combustion engine and a method of manufacturing it

ABSTRACT

A tappet in a direct acting type valve operating mechanism of an internal combustion engine comprises a top wall and a cylindrical portion. The upper and lower surfaces of the top wall contacts a rotary cam and the end of a stem of an engine valve respectively. A hard metal layer is formed at the upper and lower surfaces of the top wall to increase strength thereof, thereby increasing wear resistance and durability of the tappet.

This is a division of application Ser. No. 08/422,077, filed on Mar. 10,1995 now U.S. Pat. No. 5,537,744.

BACKGROUND OF THE INVENTION

The present invention relates to tappet in use for a direct acting typevalve operating mechanism in an internal combustion engine.

In FIG. 7, "A" denotes a conventional tappet in use for a direct actingtype valve operating mechanism an internal combustion engine. The tappet"A" comprises a cylinder closed by a top wall 22 at the upper end, andin the middle of the lower surface of the top wall 22, there isintegrally formed a projection 23 which contacts the end of a stem of anengine valve (not shown). The tappet "A" is generally molded by coldforging of low carbon mild steel, and is then subjected to heattreatment such as cementation and hardening.

However, when the conventional tappet "A" as above is subjected to heattreatment such as cementation and hardening, thermal expansion occurs,thereby decreasing accuracy in size of each portion, which involves highcost for mechanically cutting and processing. Further, it requires anumber of mechanical processing steps which is troublesome, therebydecreasing productivity.

In the top wall of the tappet "A" which is molded from mild steel, thereis low strength at the lower surface which contacts the end of the stemof the engine valve and at the upper surface which contacts a rotary cameven though each portion is strengthened with cementation and hardening.Thus, compression load repeatedly acts up and down to each portion,thereby deforming it locally and increasing wear rate.

SUMMARY OF THE INVENTION

According to the present invention, to overcome the disadvantages, it isan object to provide a tappet in an internal combustion engine havinghigh wear resistance and durability by increasing strength of the lowerand upper surfaces of a top wall.

It is a further object of the present invention to provide a method ofmanufacturing a tappet which has high wear resistance and durability.

According to one aspect of the present invention, there is provided atappet in an internal combustion engine, the tappet comprising a topwall and a cylindrical portion, a hard metal layer being provided atlower and upper surfaces of the top wall, the lower surface contactingan end of a stem of an engine valve, the upper surface contacting arotary cam.

According to another aspect of the present invention, there is provideda method of manufacturing a tappet in an internal combustion engine, themethod comprising the steps of filling hard metal powder into a gap inthe middle of a die to compress it to form first compressed powdermaterial having a certain thickness; filling soft metal powder onto thefirst compressed powder material and into an annular gap to compress itto form second compressed powder material which comprises a top wall anda cylindrical portion, the first compressed material being combined to alower surface of the top wall; filling hard metal powder onto an uppersurface of the top wall of the second compressed powder material tocompress it; and sintering it to form the tappet.

According to further aspect of the present invention, there is provideda method of manufacturing a tappet by a molding device which comprises adie; a cavity in the die; an upper punch which fits and goes down in thecavity; a first lower punch which fits and goes up in the cavity,opposing the upper punch; a second lower punch which fits and goes downin the first lower punch; and a third lower punch which fits and goesdown in the second lower punch, the method comprising the steps offilling hard metal powder into a gap which is formed by lowering thethird lower punch in the second lower punch to compress it on the thirdlower punch by lowering the upper punch to form a lower projection;filling soft metal powder into an annular gap formed by the loweredfirst lower punch and an inner wall of the cavity and on a top of thesecond lower punch to compress it by lowering the upper punch andraising the first to third lower punches to form a compressed materialwhich comprises a top wall and a cylindrical portion, the lowerprojection being combined to a lower surface of the top wall; fillinghard metal powder on the top wall of the comressed material to compressit by lowering the upper punch; and sintering it to form the tappet.

According to still further aspect of the present invention, there isprovided a method of manufacturing a tappet in an internal combustionengine, filling hard metal powder into a gap in the middle of a die,soft metal powder into a cavity in a die on the hard metal powder, andfurther hard metal powder onto the soft metal powder to form a threelayer powder material; compressing the three layer powder material; andsintering it to form the tappet.

The advantages of the invention are as follows:

a) The hard metal layers are provided on the upper and lower surfaces ofthe top wall of the tappet body to increase strength thereof, therebyincreasing wear resistance and durability.

b) The tappet body is molded by powder metallurgy, so that thecomposition of powder material is optionally selected, thereby easilymanufacturing a tappet having desired properties.

c) The manufacturing process is simplified and accuracy in size isimproved, thereby minimizing cost and increasing productivity.

d) The tappet body after sintering still is microporous, therebyincreasing oil-maintenance capability and lubricating properties.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become moreapparent based on the following description with respect to appendeddrawings wherein:

FIG. 1 is a central vertical sectional view of one embodiment of atappet according to the present invention;

FIG. 2 is a central vertical sectional view which shows the step ofmanufacturing the tappet according to the present invention, in whichhard metal powder for forming a projection is filled;

FIG. 3 is a central vertical sectional view in which powder material forthe projection is molded;

FIG. 4 is a central vertical sectional view in which soft powdermaterial is filled in a gap for forming a cylindrical portion and in acavity for a top wall;

FIG. 5 is a central vertical sectional view in which the soft powdermaterial is compressed to form compressed cylindrical powder materialwhich has a bottom;

FIG. 6 is a central vertical sectional view which shows the step offilling hard powder material onto the upper surface of the top wall tocompress it; and

FIG. 7 is a central vertical sectional view which shows a conventionaltappet.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a tappet body "A" comprises a top wall 1 and a cylindricalportion 2. In the middle of the lower surface of the top wall 1, thereis fixed a smaller diameter rigid metal projection 3 which contacts theend of a stem of an engine valve (not shown).

On the upper surface of the top wall 1, a thinner rigid metal camreceiving plate 4 is fixed. The projection 3 and the cam receiving plate4 are made of rigid metal such as Fe-C-Cr-Mo-W-V, while the otherportions are made of relatively soft metal such as Fe-C.

A method of manufacturing the tappet body "A" will be described withrespect to FIGS. 2 to 6 which are central vertical sectional viewsshowing the steps of manufacturing the tappet body "A" in order.

A molding device will be described as below. In the outermost portion ofa cavity 8 in a die 5, there is provided a first thinner cylindricallower punch 7 which has an opening at the top and is slidable up anddown. In the first lower punch 7, there is provided a second lower punch8 which has a guide bore 8a on an axis and is slidable up and down. Anannular gap "C" between the inner wall of the cavity 8 and a secondlower punch 8 is set to have thickness nearly equal to that of thecylindrical portion. In the guide bore 8a, a third lower punch 9 isengaged to go up and down. The first to third lower punches 6, 8 and 9are capable of going up and down separately. 10 denotes an upper punchto be inserted into the cavity 6 and is provided above the die 5 to goup and down.

FIGS. 2 and 3 illustrate the steps in which the projection 3 of thetappet body "A" is formed by compressed powder material. First, as shownin FIG. 2 the third lower punch 9 is lowered to form a gap 9a at thetop, and hard metal powder 11 made of Fe-C-Cr-Mo-W-V is filled in thegap 9a. Then, the upper punch 10 is lowered until it is engaged with theupper end of the second lower punch 8, and at the same time, the thirdlower punch 9 is raised to compress the metal powder 11, thereby forminga primary compressed powder material 11' somewhat thicker than theprojection 3.

As shown in FIG. 4, after the compressed powder material 11' is molded,the upper punch 10 is raised, and soft metal powder 13 made of Fe-C isfilled in the annular gap "C" and the cavity 6 on the second lower punch8. Then, as shown in FIG. 5, the upper punch 10 is lowered, while thefirst, second and third lower punches 7, 8 and 9 are raised to a certainheight to compress the soft metal powder 13, thereby forming compressedpowder material 12 which comprises a cylindrical portion 12a and a topwall 12b. At the same time, the compressed powder material 11' molded inthe former step is further compressed to form a secondary compressedpowder material 11" which is nearly the same as the projection 3, andthe powder material 11' is mounted to the lower surface of the top wall12b of the compressed powder material 12 under pressure.

Then, in the step as shown in FIG. 8, the upper punch 10 is raised to acertain position, and a certain amount of hard metal powder 13 similarto the above is filled onto the upper surface of the compressed powdermaterial 12 in the cavity 6. Then, the upper punch is lowered tocompress the metal powder 13, so that thinner compressed powder material13' having the same form as the cam receiving plate 4 is mounted on theupper surface of the top wall 12b under pressure, thereby forming thecompressed powder material having a bottom and roughly the same form asthe tappet body as shown in FIG. 1. The compressed powder materialmolded by the foregoing procedures is sintered to form the tappet body"A" as shown in FIG. 1.

The, tappet body "A" in the foregoing embodiment has higher strength atthe projection 3 which contacts the end of the stem of the engine valveand at the top wall 1 which contacts the rotary cam, thereby providinghigh wear resistance to increase duration of the tappet body "A".

Powder metallurgy is applied to manufacturing of the tappet body "A", sothat strength of essential portions and accuracy in size are increasedby selecting powder material, thereby minimizing cost after sintering toincrease productivity.

In the tappet body "A" of the foregoing embodiment, the lower surface ofthe top wall 1 may be flat without the projection 3. In the variation,the top wall 1 comprises three layers in which upper and lower layersare sintered as hard compressed powder layers.

In the foregoing method of manufacturing, compressed powder materialsimilar to the tappet body "A" is formed through a plurality of steps.In FIG. 2, after the hard metal powder 11 is filled, soft powder metalis filled into the upper portion and the gap "C", and hard metal powderis further filled on the filled soft metal powder to form three layers.The metal may be compressed once with upper and lower punches. Also, thefirst and second punches 7 and 8 may be made integrally with the die 5.

The foregoing merely relates to embodiment of the invention. Variouschanges and modifications may be made by person skilled in the artwithout departing from the scope of claims wherein:

What is claimed is:
 1. A tappet in an internal combustion engine, thetappet comprising a top wall and a cylindrical portion, a smallerdiameter projection being provided in the middle of a lower surface ofthe top wall, hard metal layer being provided on a portion of the topwall which contacts a rotary cam, the smaller diameter projection whichcontacts an end of a valve being made of hard metal layer, the smalldiameter projection forming a first compressed powder material layer ofhard metal, the top wall and the cylindrical portion forming a secondcompressed powder material layer of soft metal, and the upper surface ofthe top wall forming a third compressed powder material layer of hardmetal wherein the first, the second and the third compressed powdermaterial layers are sintered.